Left atrial free ball thrombus (LABT) after mitral valve replacement (MVR) is very rare, and sudden death may occur by thrombus impaction to the mitral valve orifice. A 81-year-old woman who underwent MVR and tricuspid annuloplasty ten years ago presented with syncope. She was admitted to a hospital, and echocardiography revealed a LABT. When she took sitting position, she fainted. The free ball thrombus possibly impacted mitral valve orifice. She was transferred to our hospital and an emergent operation was performed. There was a LABT of 4 cm in diameter, which was removed. Postoperative course was uneventful. There are 12case reports which described LABT after MVR, and anticoagulant therapy was insufficient in most of those cases. Strict anticoagulant therapy is important to prevent left atrial thrombus after MVR.